1. Field of the invention
The present invention relates to a mounting substrate on which a component is mounted by soldering, and a microphone mounted thereon.
2. Description of the Related Art
FIG. 1 shows an exemplary configuration of a conventional microphone disclosed in Japanese Patent Application Laid Open No. 2003-153392 (Patent Reference 1). This microphone is an electret condenser microphone that comprises a capsule 1, and a diaphragm ring 3, a diaphragm 4, a spacer 5, a back electrode 6, a holder 7, a gate ring 8, a circuit board 10 and an electric circuit 9 mounted on the circuit board 10, which are stacked in the capsule 1 in this order from a sound aperture 2. An edge of the capsule 1 is caulked onto the circuit board 10, thereby fixing the incorporated components in the capsule 1. Conductor parts (not shown in FIG. 1) are formed on the outer surface of the circuit board 10, and a solder bump electrode 11 to constitute an output terminal and a solder bump electrode 12 to constitute a grounding terminal are formed.
FIG. 2 shows exemplary electrodes of a mounting substrate on which the microphone shown in FIG. 1 is to be mounted. As shown in FIG. 2, on a mounting substrate 13 on which the microphone is to be mounted, a circular solder part 14 corresponding to the solder bump electrode 11 of the microphone and a ring-shaped solder part 15 corresponding to the solder bump electrode 12 are formed.
When mounting the microphone on the mounting substrate 13, the solder bump electrodes 11 and 12 of the microphone are placed on the solder parts 14 and 15 of the mounting substrate 13, respectively. Then, the mounting substrate and the microphone are placed in a reflow oven to make the solder molten, thereby fusing the solder parts 14 and 15 with the solder bump electrodes 11 and 12, respectively.
Here, the solder bump electrodes 11 and 12 of the microphone are formed in the procedure shown in FIG. 3A, FIG. 3B, and FIG. 3C. As shown in FIG. 3A, a circular conductor part 10a corresponding to the output terminal and a ring-shaped conductor part 10a corresponding to the grounding terminal are formed on the outer surface of the circuit board 10 of the microphone. As shown in FIG. 3B, a resist film 10b is formed on the conductor parts 10a excluding the areas on which the solder bump electrodes 11 and 12 are to be formed. As shown in FIG. 3C, the solder bump electrodes 11 and 12 are formed on the areas in which no resist film 10b is formed in such a manner that the solder bump electrodes 11 and 12 protrude beyond a caulked part 1a. 
As described above, the reflow oven is used in the process of mounting the microphone on the mounting substrate and achieving soldering. According to the prior art, before soldering, the solder bump electrodes 11 and 12 protruding beyond the caulked part have to be formed on the microphone, and the solder parts 14 and 15 have to be formed on the mounting substrate 13. In this case, the solder has to be applied over a wide area, and an increased amount of solder is required to form such raised solder bumps. Thus, there is a problem that a large amount of solder is required to achieve adequate soldering.
In addition, in the reflow oven, the solder bump electrodes 11 and 12 and the solder parts 14 and 15 are made molten, and the solder flows over a wide area. Thus, the resist film 10b also comes into contact with the molten solder. In this way, a wide area of the circuit board 10 comes into contact with the molten solder, so that the heat is likely to be conducted to the interior of the microphone through the circuit board 10. The heat once conducted to the interior of the microphone causes a stress therein and adversely affects the same. For example, the sensitivity of the electret may be reduced.